Conformational changes of a-lactalbumin adsorbed at oil-water interfaces: interplay between protein structure and emulsion stability

Jiali Zhai, Soren V Hoffmann, Li Day, Tzong-Hsien Lee, Mary Ann Augustin, Marie-Isabel Aguilar, Tim Wooster

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Abstract

The conformation and structural dimensions of alpha-lactalbumin (alpha-La) both in solution and adsorbed at oil-water interfaces of emulsions were investigated using synchrotron radiation circular dichroism (SRCD) spectroscopy, front-face tryptophan fluorescence (FFTF) spectroscopy, and dual polarization interferometry (DPI). The near-UV SRCD and the FFTF results demonstrated that the hydrophobic environment of the aromatic residues located in the hydrophobic core of native alpha-La was significantly altered upon adsorption, indicating the unfolding of the hydrophobic core of alpha-La upon adsorption. The far-UV SRCD results showed that adsorption of alpha-La at oil-water interfaces created a new non-native secondary structure that was more stable to thermally induced conformational changes. Specifically, the alpha-helical conformation increased from 29.9 in solution to 45.8 at the tricaprylin-water interface and to 58.5 at the hexadecane-water interface. However, the beta-sheet structure decreased from 18.0 in solution to less than 10 at both oil-water interfaces. The DPI study showed that adsorption of alpha-La to a hydrophobic C18-water surface caused a change in the dimensions of alpha-La from the native globule-like shape (2.5-3.7 nm) to a compact/dense layer approximately 1.1 nm thick. Analysis of the colloidal stability of alpha-La stabilized emulsions showed that these emulsions were physically stable against droplet flocculation at elevated temperatures both in the absence and in the presence of 120 mM NaCl. In the absence of salt, the thermal stability of emulsions was due to the strong electrostatic repulsion provided by the adsorbed alpha-La layer, which was formed after the adsorption and structural rearrangement. In the presence of salt, although the electrostatic repulsion was reduced via electrostatic screening, heating did not induce strong and permanent droplet flocculation. The thermal stability of alpha-La stabilized emulsions in the presence of salt is a combined effect of the electrostatic repulsion and the lack of covalent disulfide interchange reactions. This study reports new information on the secondary and tertiary structural changes of alpha-La upon adsorption to oil-water interfaces. It also presents new results on the physical stability of alpha-La stabilized emulsions during heating and at moderate ionic strength (120 mM NaCl). The results broaden our understanding of the factors controlling protein structural change at emulsion interfaces and how this affects emulsion stability.
Original languageEnglish
Pages (from-to)2357 - 2367
Number of pages11
JournalLangmuir
Volume28
Issue number5
DOIs
Publication statusPublished - 2012

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